Relative Motion

[Klaynos]

Lets just take a moment to consider relative measurements...

 

I'm on a train an throw a ball forward at 10m/s

 

To me be ball is moving at 10m/s

 

But compared to the earth the train is moving at 20m/s

 

So compared to my friend Bill stood at the station I've thrown the ball at 30m/s.

 

So even classically velocity measurements are all relative. One of the things that comes with more advanced physics is the lack of an absolute frame of reference, and another is that velocities don't add in the way they do classically as above.

[Motor Daddy]

Lets just take a moment to consider relative measurements...

 

I'm on a train an throw a ball forward at 10m/s

 

To me be ball is moving at 10m/s

 

But compared to the earth the train is moving at 20m/s

 

So compared to my friend Bill stood at the station I've thrown the ball at 30m/s.

 

So even classically velocity measurements are all relative. One of the things that comes with more advanced physics is the lack of an absolute frame of reference, and another is that velocities don't add in the way they do classically as above.

 

Did you forget that the initial velocity of the ball is 20m/s? If you threw the ball in the same direction of travel of the train, you accelerated the ball an additional 10 m/s, is all you did. It had an initial velocity of 20m/s, and a final velocity of 30 m/s.

[Klaynos]

Not from the perspective of me stood on the train it's not. It's 0...

[Cap'n Refsmmat]

He's standing on a train moving 20m/s and lobs the ball at 10m/s in the direction of travel of the train. A guy standing inside the train with a radar gun will record the ball traveling at 10m/s. A guy standing on the platform will record the ball traveling at 30m/s.

[Motor Daddy]

Not from the perspective of me stood on the train it's not. It's 0...

 

If the train is moving 20m/s, and the ball is 0 m/s relative to the train, the ball is also traveling 20m/s, and so are you.

 

He's standing on a train moving 20m/s and lobs the ball at 10m/s in the direction of travel of the train. A guy standing inside the train with a radar gun will record the ball traveling at 10m/s. A guy standing on the platform will record the ball traveling at 30m/s.

 

The radar gun on the train has an initial velocity of 20 m/s.

 

If the ball is sitting on the table on a train traveling a velocity of 20m/s, how fast will the people at the station measure the ball on the table as the train passes at 20 m/s??

Edited June 20, 2008 by Motor Daddy
multiple post merged

[Cap'n Refsmmat]

If the train is moving 20m/s, and the ball is 0 m/s relative to the train, the ball is also traveling 20m/s, and so are you.

Then the ball is thrown at 10m/s.

 

The radar gun on the train has an initial velocity of 20 m/s.

Yes. The point is that this works at slow speeds. But the same "the train's going 20m/s, and I threw the ball at 10m/s, so it's going 30m/s" addition does not work at high speeds. Special relativity steps in and makes things complicated.

[Klaynos]

If the train is moving 20m/s, and the ball is 0 m/s relative to the train, the ball is also traveling 20m/s, and so are you.

 

 

 

The radar gun on the train has an initial velocity of 20 m/s.

 

Only relative to the earth. It's a false absolute frame you're setting. There's NO way to tell whether the train is moving at 20m/s or the earth is moving at 20m/s

[iNow]

Swansont did a post about this just a few hours ago. Interesting video. Not related to the speed of light, but might clear up some of these subluminal relativistic effects which aren't being understood:

 

http://blogs.scienceforums.net/swansont/archives/466

 

 

... which directs readers to a video here:

http://www.popsci.com/breakdown/article/2008-06/international-experiments-relative-motion

[swansont]

Split off from the time dilation thread http://www.scienceforums.net/forum/showthread.php?t=33235

[Motor Daddy]

Only relative to the earth. It's a false absolute frame you're setting. There's NO way to tell whether the train is moving at 20m/s or the earth is moving at 20m/s

 

The people at the train station standing on the Earth measure the train at 20m/s. If you want to include the Earth's velocity than you need to tell me what initial velocity the people at the train station have.

 

You said the people measure the train at 20m/s. Don't change the story afterward.

[ydoaPs]

The people on the train measure the people at the station as traveling at 20 m/s.

[Motor Daddy]

Then the ball is thrown at 10m/s.

 

 

Yes. The point is that this works at slow speeds. But the same "the train's going 20m/s, and I threw the ball at 10m/s, so it's going 30m/s" addition does not work at high speeds. Special relativity steps in and makes things complicated.

The ball is ACCELERATED from 20m/s to 30m/s. The ball was at rest at 20m/s. It had an initial velocity of 20m/s. It has already been established.

[ydoaPs]

It had an initial velocity of 20m/s. It has already been established.

 

Not from the reference frame of the train.

[Motor Daddy]

Not from the reference frame of the train.

 

The train's velocity is 20m/s, and the ball is traveling with the train at 20m/s.

[ydoaPs]

Motor, you forgot to add the rotational velocity of the Earth's surface. And you forgot the Earth's orbit. And you forgot the Solar system's orbit in the Milky Way. You also forgot the Milky Way's motion in the Local Group. etc. That train is booking it.

 

There is no such thing as an absolute reference frame.

[Motor Daddy]

The people on the train measure the people at the station as traveling at 20 m/s.

 

So you're saying the train's velocity is 0 m/s, then?

[ydoaPs]

So you're saying the train's velocity is 0 m/s, then?

 

In one reference frame.

[Motor Daddy]

Motor, you forgot to add the rotational velocity of the Earth's surface. And you forgot the Earth's orbit. And you forgot the Solar system's orbit in the Milky Way. You also forgot the Milky Way's motion in the Local Group. etc. That train is booking it.

 

There is no such thing as an absolute reference frame.

 

I didn't forget to add squat! You TOLD ME the train has a velocity of 20m/s. Make up your mind, what is the velocity of the train???

 

In one reference frame.

 

So the train has a velocity of 0 m/s. The ball is accelerated to 10 m/s.

[Cap'n Refsmmat]

If I'm standing on top of the train and point my radar gun at the train, I'll measure it to be moving at 0 m/s. A guy standing at the platform will measure it to be moving at 20m/s. A guy in orbit around the Earth on the space shuttle will measure it to be going 7600m/s. A guy floating around in space might measure it to be going 593,294m/s. They're all right about its speed compared to them. (Add 10m/s to all those numbers to get what they'd measure the speed of the thrown ball to be.)

 

The point is that you can't define an "absolute" speed because you have to define what you're going to measure its speed against.

[Motor Daddy]

If I'm standing on top of the train and point my radar gun at the train, I'll measure it to be moving at 0 m/s. A guy standing at the platform will measure it to be moving at 20m/s. A guy in orbit around the Earth on the space shuttle will measure it to be going 7600m/s. A guy floating around in space might measure it to be going 593,294m/s. They're all right about its speed compared to them. (Add 10m/s to all those numbers to get what they'd measure the speed of the thrown ball to be.)

 

The point is that you can't define an "absolute" speed because you have to define what you're going to measure its speed against.

 

I'm not the one who said the train's velocity is 20m/s. Since you already established a velocity, I have to play by those rules. The train is traveling 20m/s, so is the ball. The ball is then accelerated to 30m/s.

[ydoaPs]

Motor Daddy read the OP for the problem statement.

[Cap'n Refsmmat]

Right. And that's the way classical physics works. But you have to agree that an astronaut armed with a radar gun would give the numbers I stated above, because he's measuring the velocity of the train compared to his space shuttle.

 

But when you venture into relativity, you can't just say "it's going 20m/s." In relativity, all reference frames are equal, meaning the astronaut is just as justified in measuring the speed of the train as 7600m/s as you are measuring it as 20. You're both correct in your measurements.

[ydoaPs]

As swansont once said, asking for a speed without a reference is like asking the difference between a duck.

[Motor Daddy]

Right. And that's the way classical physics works. But you have to agree that an astronaut armed with a radar gun would give the numbers I stated above, because he's measuring the velocity of the train compared to his space shuttle.

 

But when you venture into relativity, you can't just say "it's going 20m/s." In relativity, all reference frames are equal, meaning the astronaut is just as justified in measuring the speed of the train as 7600m/s as you are measuring it as 20. You're both correct in your measurements.

 

So when I look at a full moon at night from the Earth, it appears to me to be about 1 foot in diameter. If there was a astronaut on the edge of the left edge of the moon from my perspective, and he travels to the right edge of the moon in 12 hours (my time), did he travel .083 ft/hr?

 

As swansont once said, asking for a speed without a reference is like asking the difference between a duck.

 

And likewise, giving a speed means that that speed has a reference point for all to play the game by those rules. You can't switch the reference point and still use that given velocity in the middle of the game.

Edited June 20, 2008 by Motor Daddy
multiple post merged

[Cap'n Refsmmat]

No. You're measuring his velocity wrong. The moon isn't 1 foot in diameter.

 

Let's say you went to an astronomical observatory and measured the angle between his position in the telescope at the start of the journey and at the end of the journey, then used trigonometry to produce a number for his speed. It would vary greatly from the number his speedometer stated -- because not only is he moving across the moon, but the moon is moving across the sky. In that case, you are both correct about his velocity.

 

And likewise, giving a speed means that that speed has a reference point for all to play the game by those rules. You can't switch the reference point and still use that given velocity in the middle of the game.

 

That is indeed a key point.